Japanese Patent Application Laid-Open Publication No. 2005-190706 discloses a fuel cell stack structure in which multi-cell modules each formed by stacking multiple fuel cells are arranged in a row in a cell-stacking direction, and a space between adjacent ones of the cell modules is sealed with a bead gasket. A separator of each end cell of a multi-cell module to be in contact with the bead gasket is given larger surface rigidity than the separators of center cells of the multi-cell module. Specifically, in order for the separator of the end cell of the multi-cell module to have the larger surface rigidity than those of the center cells, a flat panel is superimposed on the separator of the end cell.